The field of the invention is polycarboxylic acid anhydrides prepared by oxidation of aromatic compounds.
The invention is particularly related to a method and apparatus for removing deposits from fixed-bed catalysts mounted in the tubes of multi-tube reactors (tube-nest or tube-cluster reactors). The removal of deposits restores the output to the levels prior to deposition.
The present invention is described in relation to the embodiment of the preparation of the anhydride of phthalic acid, however, the invention is effective wherever fixed-bed catalysts are mounted in multi-tube reactors and depositions occur in the course of the process.
Specifically the process for preparing the anhydride of phthalic acid is considered, wherein the crude naphthalene following tension evaporation in the presence of fixed-bed catalysts containing vanadium oxide is oxidized by means of gases containing oxygen, preferably air, to the anhydride of phthalic acid.
The state of the art of low-temperature fixed-bed air oxidation of naphthalene may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", Vol. 15 (1968), pages 448-457, and U.S. Pat. Nos. 1,285,117; 1,787,416; 1,787,417; 1,971,888; and 3,112,324, the disclosures of which are incorporated herein.
As pointed out by Kirk-Othmer, Vol. 15, at page 450, the low temperature fixed-bed air oxidation of naphthalene is carried out at 350.degree. - 360.degree.C with 4-5 second contact time. The catalyst is V.sub.2 O.sub.5 on silica with 20-30 percent potassium sulfate.
When phthalic anhydride is manufactured on an industrial scale, crude naphthalene is used for economical considerations in lieu of pure naphthalene as the starting material, preferably being of the so-called hot-press material quality. The purity of this crude naphthalene is characterized by the freezing point, which fluctuates between 78.degree. and 79.degree.C and indicates a naphthalene content approximately between 95.5 and 97.5 percent. Coal tar derived crude naphthalene contains appreciable amounts of sulfur compounds, especially thionaphthene, as contaminations. It further contains phenolic substances and nitrogen compounds in varying amounts, also admixtures of slight amounts of inorganic compounds in the form of iron and alkali salts and residues of higher boiling points.
Because of these contaminants, crude naphthalene is generally tension vaporized in a partial stream of the reaction air, temperature being set between 130.degree. and 160.degree.C. During this tension vaporization, i.e., during a vaporization into the gas flowing over the crude naphthalene, not only does the concentration of the present impurities with higher boiling points increase in the vaporizer's sump, but also new amounts are formed by oxidation and resinification. These impurities may not be volatile at all, and furthermore, partly acid compounds may be formed as well. After a few weeks, the concentration of these substances in the sump of the tension vaporizer will rise to 50 percent and beyond before these residues are sluiced out together with the naphthalene still present.
Increasing tendency to foaming is observed as the naphthalene content drops in the vaporizer sumps, the air-stirred foam filling the empty space and the vaporizer components above the liquid level.
Gases containing oxygen, and in practice air alone, are used for the gaseous oxidation of naphthalene to the anhydride of phthalic acid, these gases streaming through the reactor from top to bottom while being laden with naphthalene. As a rule, supported catalysts are used, which will mainly support vanadium pentoxide as the effective substance. As regards the supports, one must distinguish between those substances which are inert in the sense of the reaction, for instance silicon carbide, porcelain or corundum, and those supports which control the catalytic activity of vanadium pentoxide, for instance silica gel or titanium dioxide in modifying anatase. Conventional activating additives are used in this respect too, for instance potassium pyrosulfate, by means of which the temperatures required for oxidation are lowered by approximately 80.degree.-100.degree.C. It is especially the last-named catalysts, the so-called "german-type contacts," which possess very long life besides high selectivity.
Despite this long catalyst life and even in the absence of selectivity degradation, the prior art catalysts have to be exchanged every few years because the flow resistance in the reactor pipes is so increased that the flow rate possible by means of the reaction air blowers falls below the economical values. There is even danger in the absence of catalyst exchange that the naphthalene transmission drops to such an extent that the heat of reaction no longer covers the radiative emission from the furnace and that therefore oxidation is no longer feasible.
Such a change of catalyst, which in this instance is required because of a decrease in activity, means not only interrupting manufacture for several weeks, but it also is very costly. Because of the great care required in filling thousands of tubes and in setting these to the same pressure gradient, catalyst change demands a great deal of labor.
On the basis of prior experience with other catalytic methods, expert opinion to date has always assumed that the cause of the increase in resistance of the reactor tubes filled with catalysts was due to the formation of catalyst dust or to the baking of the catalyst grains by means of deposited and incrusted reaction products.